Why traffic light signals Red Yellow Green?

TRAFFIC LIGHT SIGNALS

Traffic lights is a set of automatic,coloured lights for controlling traffic.
Traffic lights are signaling devices positioned at road intersection, pedestrian crossing , and other locations to control the flows of traffic.

We all have stuck in traffic jam and observed the traffic lights which give us signal :
         Red - Stop,when to stop
         Yellow (Amber) - Caution , go slow 
         Green -go, move  by changing colour. 

What is the reason behind the use of Red , Yellow and Green in traffic light signals ?

Red was considered as the signal of danger from ages due to colour of Blood and Fire since long. Red colour can be seen from large distance due to its longest wavelength in visible spectrum and less scattering character.
Yellow has wavelength next to red colour so it has also least scattering properties and can be seen from large distances.Yellow light can be easily visible in night and it also offer most striking contrast between Red and Green colour.
Green colour has wavelength next to yellow and also scatter least.

Following diagram show visible colour em spectrum VIBGYOR increasing wavelength left to right.

Em Spectrum of Visible Light

Violet colour scatter 10 times more than Red colour.

This scheme of colour is used universal in almost all countries in world for traffic light signals.